A press apparatus as shown in FIG. 13 is used conventionally to remove water from a wet paper web in a papermaking process. The press apparatus comprises a pair of press rolls P, and a pair of press felts 12 which support a wet paper web. The press apparatus squeezes water from a wet paper web W by applying pressure, by means of the rolls P, to the press felts 12 and, through the felts, to the wet paper web W. Water squeezed from the wet paper web W is absorbed by the press felts 12. Each of the press felts 12 comprises a base body for maintaining strength, and a batt layer on both sides of the base body. The base body and the batt layer are intertwiningly integrated by needle punching.
FIG. 14 is an enlarged view showing the nip of the press part of FIG. 13 in order to illustrate and explain the transfer of water squeezed from the wet paper web W. The details of the structure of the press felts 12 are not shown in this figure. When the press rolls P rotate in the direction of the arrows in FIG. 13, the press felts 12 and the wet paper web W are moved in the directions indicated by the arrows as they pass between the press rolls P. The press felts 12, and the wet paper web W are compressed in the press part, and water in the wet paper web W is squeezed and absorbed by the press felts 12. However, since pressure applied to the wet paper web W and the press felts 12 is abruptly released after the web and the felts move past the nip at the center of the press part, the volume of the press felts 12 suddenly as the felts and wet paper web move from the nip toward the exit of the press part. A negative pressure is generated in the press felts 12, and a capillary phenomenon occurs since the wet paper web W comprises fine fibers. Therefore, water absorbed by the press felts 12 is transferred back to the wet paper web. This is referred to as “re-wetting” and a well-recognized problem in a conventional press.
FIG. 15 shows a felt, described in U.S. Pat. No. 5,372,876, which is designed to prevent re-wetting. The felt 11, comprises a base body 31 and batt layers 21 on both sides of the base body 31. A hydrophobic film 41, made of a spun bond, is provided on the base body 31, and separates the press roll side layer from the wet paper web side layer. It is believed that, when this felt 11 is used, re-wetting is prevented, even when the pressure applied to the felt 11 is suddenly released, since the water absorbed in the press roll side layer is not easily transferred to the wet paper web side.
Unexamined Japanese Patent Publication No. 8888/1991 describes another approach to the re-wetting problem, in which a barrier layer is provided to prevent water, once absorbed, from being transferred to the wet paper web side.
U.S. Pat. No. 4,830,905 describes a press felt, in which a foam layer having closed cells is provided. It is believed that, when this felt is used, re-wetting is prevented since water is held in the cells.
Despite the above measures, a problem remains because, in the felts disclosed in U.S. Pat. No. 5,372,876 and Japanese Patent Publication No. 8888/1991, it is difficult to prevent the transfer of water since a hydrophobic film having a great number of apertures, and a porous film are used, respectively. In the case of U.S. Pat. No. 4,830,905, there is the problem of discharging water from the cells of the foam layer.